Critical rainfall is an important indicator of flash flood disaster early warning. In order to analyze the effect of rainfall pattern on thecritical rainfall in small watershed， this paper classified and divided the rainfall process in the basin according to the different locations of the rainpeaks， and qualitatively analyzed all kinds of typical rainfall patterns. On this basis， the Pilgrim&Cordery method was used to quantitatively determinethe various typical rain basin type distribution of the schedule， and through the test algorithm to compute the corresponding critical rainfall.Taking advantage of the recent floods in the basin， the influence of the rainfall pattern on the critical rainfall was further analyzed anddemonstrated by using the critical rainfall deviation index. This paper studied the application of the small watershed of the Peihe River in XinxianCounty， Henan province. The results show that the design of rain patterns can not fully represent the type of rainfall in the basin， which belongs tothe rain peak in the back form， the critical rainfall determined should be 33% different from the rain peak in the former form that is often presentin the river basin， there is a large difference in critical rainfall of multi peak rain type， with a difference of 43%. According to real-time rainfalltype， the corresponding critical rainfall can be used to improve the warning level of flash flood disaster.